The present invention relates to an optical recording medium in which recording and reproduction can be effected by means of a light beam.
As optical recording media or materials for optical disks, optical cards, optical tapes, etc., there have been heretofore known rare earth-transition metal alloy films, films of reducing oxides such as chalcogen compounds utilizing phase transition between amorphous and crystalline states, heat mode recording media, thermoplastic recording media, etc. More specifically, as the rare earth-transition metal optomagnetic recording media, there have been known polycrystalline films such as those of MnNi and MnCuBi; amorphous films such as those of GdCe, GdFe,TbFe, DyFe, GdTbFe, TdDyFe, GdFeCo, TbFeCo and GdTbCo; and single-crystalline films such as that of GdIG.
Among these optical recording media in the form of films, the above-mentioned amorphous film has been recently considered excellent in view of such factors as film formability for producing a large area of film around room temperature, writing efficiency for writing signals at a small optical heat energy, and readout efficiency for reading out written signals at a good S/N ratio. Especially, GdTbFe has been considered to be the optimum optical heat recording medium because it has a large Kerr rotation angle and a Curie point at around 150.degree. C.
However, the optical recording medium generally has a drawback that it is poor in corrosion resistance. Thus, when an optical recording medium is exposed to the atmosphere or moisture, it gradually has a lower recording performance, until it is completely oxidized to lose its characteristics as a recording medium. Especially, the amorphous magnetic material such as GdTbFe has a poor corrosion resistance.
In order to obviate such a drawback, there have been proposed provision of a protecting cover such as a protecting layer of, for example, SiO.sub.2 or SiO, and a disk-form recording medium sealed in an inert gas.
However, we have found that the protective effect of an oxide film such as SiO, SiO.sub.2 or Al.sub.2 O.sub.3 film is not sufficient when the optical recording medium is as thin as several hundred angstromes or below. This is considered because, when an oxide film is used, oxygen in the oxide per se is combined with the optical recording medium, and moreover the film of an oxide is porous and is easily penetrable by molecules of, e.g., water vapor.
On the other hand, a film of a nitride such as AlN or Si.sub.3 N.sub.4 has been used as a protecting layer. Such a nitride film, however, is inferior to an oxide film in adhesiveness to a plastic substrate and has a drawback that it has too high a refractive index.